This application claims priority from PCT application PCT/FR00/00044, filed Jan. 11, 2000, which is based upon French national application 99/00184, filed Jan. 11, 1999.
The present invention relates to optical fiber transmission systems.
In particular, an advantageous application of the invention lies in long-distance transmission systems, and in particular in transoceanic transmission systems which need to be capable of carrying information over distances in excess of 5000 kilometers (km).
Numerous long-distance transmission systems have already been proposed which implement return to zero (RZ) modulation, i.e. modulation in the form of pulses of duration shorter than the bit time, and which comprise two segments of dispersive fiber between successive amplifiers (e.g. constituted by erbium-doped fibers) the segments having chromatic dispersion of opposite signs and compensating each other.
In such systems, local chromatic dispersion serves to reduce non-linear interaction phenomena between channels (four-wave mixing), while compensating the chromatic dispersion of the fibers makes it possible to obtain mean chromatic dispersion that is small and consequently makes it possible to minimize phenomena of pulse spreading.
The chromatic dispersion slope can be reduced in various ways and in particular either by using small-slope fibers or by using fibers having slopes of opposite signs, or by demultiplexing the channels and compensating chromatic dispersion individually in each channel.
For descriptions of optical transmission systems implementing chromatic dispersion compensation, reference can advantageously be made to the following publications:
[1] I. Morita, K. Tanaka, N. Edagawa, M. Suzuki, xe2x80x9c40 Gbit/s single-channel soliton transmission over 10,200 km without active inline transmission controlxe2x80x9d, Post-deadline paper, p. 49, ECOC""98 (Madrid);
[2] N. Edagawa, I. Morita, M. Suzuki, S. Yamamoto, K. Tanaka, S. Akiba, xe2x80x9cLong distance soliton WDM transmission using a dispersion-flattened fiberxe2x80x9d, Post-deadline paper PD19, OFC""97 (Dallas);
[3] M. Suzuki, H. Kidorf, N. Edagawa, H. Taga, N. Takeda, K. Imai, I. Morita, S. Yamamoto, E. Shibano, T. Miyakawa, E. Nazuka, M. Ma, F. Kerfoot, R. Maybach, H. Adelmann, V. Arya, C. Chen, S. Evangelides, D. Gray, B. Pedersen, A. Puc, xe2x80x9c170 Gbit/s transmission over 10,850 km using large core transmission fiberxe2x80x9d, Post-deadline paper PD17, OFC""98 (San Jose);
[4] N. Edagawa, M. Suzuki, N. Takeda, K. Imai, S. Yamamoto, S. Akiba, xe2x80x9c213 Gbit/s. (20xc3x9710.66) over 9000 km transmission experiment using dispersion-slope compensatorxe2x80x9d, Post-deadline paper PD13, OFC""98 (San Jose);
[5] M. Murakami, T. Matsuda, T. Imai, xe2x80x9cQuarter Terabit (25xc3x9710 Gbit/s) over 9288 km WDM, transmission experiment using non-linear supported RZ pulse in higher order fiber dispersion managed linexe2x80x9d, Post-deadline paper, p. 79, ECOC""98 (Madrid);
[6] D. Le Guen, A. O""Hare, S. Del Burgo, D. Grot, F. Favre, T. Georges, xe2x80x9cNarrow band 640 Gbit/s soliton WDM transmission over 1200 km of standard fiber with 100 km-21 dB amplifier spansxe2x80x9d, Post-deadline paper, p. 61, ECOC""98 (Madrid).
Nevertheless, the systems described in publications [1-5] have the drawback of not enabling high transmission rates to be achieved because of cross phase modulation phenomena between adjacent channels preventing multiplexing being sufficiently dense, i.e. a channel separation of less than 0.8 (nanometers) (nm) for 20 gigabits per second (Gbit/s) channels, of 1.8 nm for 40 Gbit/s channels, and of 0.4 nm at 10 Gbit/s (references [1] and [2]). The system of reference [6] allows multiplexing to be dense but over distances that are too short (2000 km) and limited by interaction.
To enable high data rates to be achieved over transoceanic distances, proposals have also been made for transmission systems implement soliton type modulation.
Systems of that type are described, for example, in the following publication:
M. Nakazawa et al., xe2x80x9c16 Gbit/s WDM (20 Gbit/sxc3x978 channels) soliton transmission over 10,000 km using inline synchronous modulation and optical filteringxe2x80x9d, PD10-1, Optical Soliton Transmission Research Groupxe2x80x94NTT Access Network Systems Laboratoriesxe2x80x94Tokai, Ibaraki-ken 319-11 Japan.
However, the technique proposed in the above article is very difficult to implement because of the precision required (better than 0.1 picoseconds per nanometer per kilometer (ps/nm/km)) concerning the value of chromatic dispersion in each of the fiber segments.
In addition, when using fibers with non-zero chromatic dispersion slope, that technique allows transmission on certain wavelengths only.
The object of the invention is to mitigate the drawbacks of prior techniques and to propose a long-distance transmission system enabling high data rates with dense multiplexing and with a large passband.
British patent GB 2 299 473 discloses a long-distance optical transmission system comprising pulse emitter and receiver means and an optical line which extends between said emitter and receiver means and which comprises alternating segments of dispersive fibers having chromatic dispersion of opposite signs, and also a plurality of amplifiers, said optical lines including one pair of dispersive fiber segments having chromatic dispersion of opposite signs between successive amplifiers.
The invention provides a long-distance optical transmission system comprising pulse emitter and receiver means and an optical line which extends between said emitter and receiver means and which comprises alternating segments of dispersive fibers having chromatic dispersion of opposite signs, and also having a plurality of amplifiers, said optical line comprising at least one pair of dispersive fiber segments having chromatic dispersion of opposite signs between successive amplifiers, the system being characterized in that said optical line comprises a plurality of such pairs between successive amplifiers, and in that the cumulative dispersion C over the majority of the segments of the optical line satisfies the relationship.
xe2x80x83|C|xcex94xcexd2 less than 0.3
where C is expressed in ps/nm and where xcex94xcexd is the half-height spectral value of the pulses expressed in terahertz (THz)
In addition, the system proposed by the invention advantageously includes the various following characteristics taken singly or in any feasible combination:
the cumulative dispersion C of the segments of the optical line satisfies the relationship:
|C|xcex94xcexd2 less than 0.25
where C is expressed in ps/nm and where xcex94xcexd is expressed in THz;
the cumulative dispersion C of the segments of the optical line satisfies the relationship:
0.03 less than |C|xcex94xcexd2
where C is expressed in ps/nm and where xcex94xcexd is expressed in THz;
the chromatic dispersion of the segments of anomalous dispersion fiber is about 17 ps/nm/km at a wavelength of 1550 nm;
the chromatic dispersion of segments of normal dispersion fiber is about xe2x88x9285 ps/nm/km;
the optical line has three pairs of segments of dispersive fibers having chromatic dispersion of opposite signs, between successive amplifiers;
a segment of anomalous dispersion fiber extends over a distance of about 10 km and a segment of normal dispersion fiber extends over a distance of about 2 km;
the chromatic dispersion of the segments of normal dispersion fiber is about xe2x88x9217 ps/nm/km;
the optical line has two pairs of segments of dispersive fibers having chromatic dispersion of opposite signs between successive amplifiers; and
a segment of fiber extends over a distance of about 10 km.